1. Technical Field
The present invention relates to a light guide unit emitting light from a surface of a light guide plate, a light source device including the light guide unit, and a display apparatus including the light source device.
2. Description of the Related Art
Mobile phones such as smartphones and the like, tablet terminals, personal computers, liquid crystal TVs and the like each include a liquid crystal display apparatus. A liquid crystal display apparatus includes a liquid crystal display panel, a backlight unit and the like. Backlight units are available in a direct type including a diffuser diffusing light from an LED (light emitting diode) provided at, for example, a back of a liquid crystal display panel, and an edge light type including a light guide plate located at a back of a liquid crystal display panel and an LED located to face an edge surface of the light guide plate. Light emitted from the LED enters into the light guide plate from the edge surface of the light guide plate, passes a light outgoing surface of the light guide plate and enters into the liquid crystal display panel from the rear surface of the liquid crystal display panel. On a light reflecting surface of the light guide plate opposite to the light outgoing surface, a reflection plate returning the light output from the light guide plate back to the light guide plate is disposed facing the liquid crystal display panel.
At a light reflecting surface of the light guide plate, a light scattering unit such as a dot pattern, a prism-like groove or the like is provided in order to scatter the light totally reflected inside the light guide plate and output the light from the light outgoing surface and also in order to make the luminance uniform at the light outgoing surface.
In the case where the light guide plate and the reflection plate are fixed to each other, the light reflecting surface of the light guide plate is not in contact with air having a small refractive index and thus does not fulfill the conditions required for the total reflection. As a result, the luminance at the light outgoing surface of the light guide plate is made non-uniform. In such a situation, a light guide unit including a light guide plate and a reflection plate bonded with each other via a dot pattern made of a pressure sensitive adhesive and a liquid crystal display apparatus including such a light guide unit are disclosed (see Japanese Laid-Open Patent Publication No. 2013-93195, (hereinafter “Patent Document 1”)).
In the case where a conventional liquid crystal display apparatus as disclosed in Patent Document 1 uses a light guide plate having a coefficient of linear thermal expansion larger than that of the reflection plate, the reflection plate is contracted in accordance with a temperature change. By contrast, in the case where a light guide plate having a coefficient of linear thermal expansion smaller than that of the reflection plate is used, the light guide plate is contracted in accordance with a temperature change.
FIGS. 12A to 12E provide schematic views each showing an example of expansion and contraction of a conventional light guide plate and a conventional reflection plate. FIG. 12A shows a state where the reflection plate is contracted more than the light guide plate. The reflection plate is delaminated in both of two end parts thereof. In FIG. 12A, an adhesive dot pattern is delaminated from the reflection plate (see reference signs P1 and P2). FIG. 12B shows a state where the light guide plate is contracted more than the reflection plate. The light guide plate is delaminated in a central part thereof. In FIG. 12B, the adhesive dot pattern is delaminated from the reflection plate (see reference sign P3). FIG. 12C shows a state where the reflection plate is contracted more than the light guide plate. The reflection plate is delaminated in both of two end parts thereof. In FIG. 12C, the adhesive dot pattern is delaminated from the light guide plate (see reference signs P4 and P5). FIG. 12D shows a state where the light guide plate is contracted more than the reflection plate. The light guide plate is delaminated in a central part thereof. In FIG. 12D, the adhesive dot pattern is delaminated from the light guide plate (see reference sign P6). FIG. 12E shows a case where the dot pattern has a pressure sensitive adhesivity strong enough to prevent the dot pattern from being delaminated from the light guide plate or the reflection plate, and shows a state where the reflection plate is contracted by the difference in the coefficient of linear thermal expansion between the light guide plate and the reflection plate and both of the light guide plate and the reflection plate are warped.
FIGS. 13A to 13C provide schematic views each showing an example of light propagation in a conventional light guide plate. FIG. 13A shows a state where neither the light guide plate nor the reflection plate is expanded or contracted, and the dot pattern is not delaminated. In this case, apart of the light propagating inside the light guide plate is transmitted through a light reflecting surface and is incident on the dot pattern. The dot pattern is light scattering. Therefore, the light incident on the dot pattern is scattered by the dot pattern and is incident back on the light guide plate. Namely, when a part of the light propagating inside the light guide plate is scattered by the dot pattern, the scattered light propagates toward the light outgoing surface of the light guide plate relatively uniformly. The amount of light propagating inside the light guide plate is smaller as the light is farther from a light source. However, the dot pattern is provided to have a higher density as the light is farther from the light source. Therefore, the amount of light scattered by the dot pattern is relatively large in an area far from the light source. In this manner, the luminance at the light outgoing surface of the light guide plate is made uniform.
FIG. 13B shows a state where the dot pattern is delaminated from the reflection plate as represented by reference signs P1, P2 and P3 in FIG. 12A and FIG. 12B. In this case, the dot pattern and the reflection plate are separated from each other, and the distance between the reflection plate and the light guide plate is increased. As a result, the light scattered by the dot pattern is incident on the light guide plate at diverged positions and the light is expanded. Therefore, the luminance distribution is made different from the luminance distribution that should be provided as shown in FIG. 13A, and thus a border between a bright region and a dark region is made at the light outgoing surface of the light guide plate.
FIG. 13C shows a state where the dot pattern is delaminated from the light guide plate as represented by reference signs P4, P5 and P6 in FIG. 12C and FIG. 12D. In this case, the light guide plate and the dot pattern are separated from each other, and thus the light reflecting surface of the light guide plate is put into contact with an air layer. As a result, the light is totally reflected by the light reflecting surface. Therefore, the scattered light that should be output as shown in FIG. 13A is not output. Thus, the luminance is decreased, and a portion of the light outgoing surface of the light guide plate corresponding to the separated dot pattern and the vicinity thereof becomes dark.
As described above, a conventional liquid crystal display apparatus has a problem that the difference in the coefficient of linear thermal expansion between the light guide plate and the reflection plate, for example, warps the reflection plate or sags the light guide plate, resulting in luminance non-uniformity at the light outgoing surface of the light guide plate.